Thanks for the answers guys. I'll probably get a new power supply and see if that fixes the problem (unless someone has some other idea as to what's causing the reboots), I'm thinking a 550W Chieftec should be strong enough.
As for the amount of RAM, I agree, I actually had a spare 1GB, but the thing is I have a 32-bit XP and 32-bit operating systems don't see more than 3GB of RAM. If I were to put that fourth module, I'd have to get a good 64-bit OS that can also run all my games without problems.